As demand of communication increases, optical networks using wavelength division multiplexing (WDM) have come into widespread use. Wavelength division multiplexing is a technique that transfers a plurality of optical signals with different frequencies by multiplexing them. In wavelength division multiplexing, for example, 88 optical signals that have different wavelengths and a transmission rate of 40 Gbps may be transmitted as an optical wavelength multiplexed signal (referred to below as an optical multiplexed signal) by multiplexing them.
An optical add-drop multiplexer referred to as, for example, a reconfigurable optical add-drop multiplexer (ROADM) is known as an optical transmission device using wavelength division multiplexing. A ROADM device has a transceiver for optical signals referred to as, for example, a transponder.
A ROADM device transmits, to another node, an optical multiplexed signal in which a plurality of signals with arbitrary wavelengths input from a plurality of transponders are multiplexed, and separates a signal with an arbitrary wavelength from an optical multiplexed signal received from another node and outputs it to a transponder. That is, a ROADM device inserts or branches an optical signal with an arbitrary wavelength.
A ROADM device has an optical splitter and a wavelength selective switch (WSS) for each path. A path is a transmission channel that runs toward another node in a network.
An optical splitter outputs an optical multiplexed signal input from a corresponding path, to the wavelength selective switches and transponders of the other paths. A wavelength selective switch selects, from optical multiplexed signals input from the optical splitters and transponders of the other paths, an optical signal with the wavelength to be output, multiplexes the selected signal, and outputs it as an optical multiplexed signal to a corresponding path.
When a network is extended, the number of nodes to which ROADM devices are connected increases and the number of paths to which connection is made also increases. Accordingly, a job for adding transponders, optical splitters, and wavelength selective switches to ROADM devices arises. In this job, transponders, optical splitters, and wavelength selective switches have to be interconnected via many optical fibers.
For example, Japanese Laid-open Patent Publication No. 2006-310963 discloses a technique for controlling the branching ratio of a variable optical coupler on the drop side according to an optical input level and optical output level of a ROADM device. In addition, Shinlin Xiao and seven others, “Tunable optical Splitter technology”, Proceedings of SPIE, vol. 4870, pp. 532-539, 2002 discloses a technique for a tunable optical splitter.